Due to simplicity and high efficiency, multilevel DC-DC converter systems are finding increasing applications in modern power systems, such as DC collector grids in large-scale offshore wind farms, photovoltaic (PV) plants and shipboard power systems (SPS).
As the interface, the high-voltage, high-power DC-DC converter is a key enabling technology in MIN/DC (medium voltage direct current) systems. Galvanic isolation is generally required in the DC-DC converters, not only for safety reasons, but also to establish a high voltage conversion ratio. In the DC-DC converters, high efficiency and high power density are desired, especially for SPS.
Furthermore, since the medium-voltage DC circuit breakers are not yet readily available for high power levels, the DC fault clearing has become a critical challenge in MVDC systems. As an alternative approach, a fast DC fault clearance process based on coordinated control of power converters and a mechanical contactor have been presented in the prior art. In the prior art process, the DC fault is detected by power converters, and then the fault segment is located through a differential protection scheme with assistance from the power converters. After the fault segment has been located, the system is fully dc-energized to isolate the fault segment, and then the system is re-energized for recovery. The key to such a coordinated fault management system is the power converter, which not only limits, but also provides for a controlled DC fault current, and as such, exhibits DC fault ride-though capability.
Dual-active-bridge (DAB) DC-DC converters are known in the art, including input-series output-parallel (ISOP) DAB DC-DC converters, which have the advantage of high efficiency due to zero-voltage-switching (ZVS) operation, and which exhibit high power density, high frequency isolation, and low device number. However, the DAB DC-DC converters currently known in the art lack DC fault ride-through capability and inject extra fault current as a result of the discharging of output capacitors when a DC fault occurs. The DAB concept has also been extended for modular structure, however, while the known converter can isolate a DC fault, it cannot provide controllable DC fault current.
Isolated modular multilevel DC-DC converters (IMMDC), consisting of two modular multilevel converters (MMC) in front-to-front configuration through a transformer are also known in the art. Based on MMC, the IMMDC exhibits good performance under DC fault, and the size and volume of the passive components may be reduced with medium-frequency operation. However, the limitation of this known IMMDC converter is the relatively low efficiency and low power density, resulting from high switching loss and conduction loss.
For wind and PV generation, large power fluctuations and energy variations are unavoidable, which deteriorates the grids stability and has become a major barrier for their high penetration of the technology. Similarly, in MVDC distribution systems, there are a wide variety of loads, including high power propulsion loads and pulsed loads. These loads introduce a large amount of pulsed and ripple current in a wide frequency range, which may cause voltage oscillations on the MVDC system. In these applications, energy storage is essential to smooth the power fluctuations and to stabilize the grid.
Batteries offer unique and scalable energy storage solutions for high-power and long-term energy demands in the power range of up to several hundred megawatts. At the interface between the battery energy storage and the MVDC bus, the BESS (battery energy storage system) converter is a key enabling technology and is required to operate in both high voltage and high power ratings. Galvanic isolation is additionally required, not only for safety reasons, but also to provide a high voltage conversion ratio. In addition, due to the relatively high cost and limited cyclic life of battery units, the BESS converter must be highly efficient in order to maximize the utilization of the battery. Moreover, as DC circuit breakers are not yet readily available for high power conditions, it is desirable that the BESS converters provide good DC fault responses to lead to a more secure and robust power transport providing inherent fault protection and DC fault ride-through capabilities.
Conventional BESSs are mainly focused on AC systems, where a line frequency transformer is usually used as an interface to medium voltage or high voltage AC grids, thus the conventional BESSs known in the art are not suitable for medium voltage or high voltage DC grid applications. Several attempts at modular BESS converters for a DC grid have been made, however they do not meet all the requirements mentioned above.
Cascaded dual-active-bridge (DAB) BESS converters are known in the art and have the advantage of a small number of devices and soft-switching operation. However, the cascaded dual-active-bridge (DAB) BESS converters known in the art lack fault ride-through capability and will inject extra fault current, as a result of the output capacitors, when a DC fault occurs. An isolated modular multilevel DC-DC converter (iMMDC) is also known in the art for BESS in MVDC applications in which DC fault ride-though is achieved by the MMC stage on the MVDC side. Although the reactive components can be reduced with medium frequency operation, the converter does not provide soft-switching operation and also utilizes a large number of switches, resulting in a potential and efficiency penalty.
Accordingly, what is needed in the art is a modular multilevel DC-DC converter that is galvanically isolated and which incorporates control methods for operating the converter to employ soft-switching and DC fault ride-though capability.